Overly Dramatic View of the Two...
I did a reading with my new Steampunk Tarot deck and it had a caution in the guidebook to be careful is five of cups appears in a reading with the moon.
I wonder if you shouldn't write to the authors of the Steampunk tarot. I'm sure they're on line and likely have at least a Facebook page. As a writer of tarot books, myself, I think it irresponsible of them to caution readers about any combo without fully explaining what it means. A guidebook is no place to be mysterious.
That said, I'd say that the authors of this deck think that these two cards, together, might indicate suicidal tendencies. 5/Cups often indicates an obsession over something you've lost or done wrong, to the point where your self worth is gone and you can't think of anything besides how miserable you are, how horrible a person you are, etc. The Moon *can be* delusions and sometimes literal "lunacy." Put these two together and you *could* get suicidal depression. You know, those articles in the news where a husband felt himself a failure as a man, or a wife was in deep postpartum and they killed the kids before killing themselves.
Yeah, I know, scary. But the guidebook is, again, being irresponsible (I think) with this dramatic view of the combo. Such mental heath issue exist in the world. And so there have to be card combinations to indicate them. But we don't read about them happening on a daily basis. Meaning 9 times out of 10, the combo is simply saying that you're sad about something and having bad dreams.
Which, yes, is enough that you ought to consider getting exercise, fresh air, and talking to someone to make yourself feel better. But while I'd ask someone I was reading for exactly how they were feeling if I got these two, just to be sure they weren't in any danger, I wouldn't jump to any scary conclusions and call up the police to take them away for 24 hours observation. That would be...irresponsible.